The present invention relates generally to exposure, and more particularly to an exposure method used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as fine contact hole patterns used for micromechanics. Here, the “micromechanics” is technology for applying the semiconductor IC fabricating technique for fabrications of a fine structure, thereby creating an enhanced mechanical system that may operate at a level of micron.
Reduction projection exposure apparatuses have been conventionally employed which use a projection optical system for projecting a circuit pattern formed on a mask (reticle) onto a wafer, etc. and for transferring the circuit pattern, in manufacturing such fine semiconductor devices as semiconductor memories and logic circuits in the photolithography technology.
The critical dimension transferable by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Smaller resolution has recently been demanded with a demand for finer semiconductor devices. only the exposure light with a small wavelength has the limit to meet this requirement, and the projection optical system is expected to improve resolution using higher NA. At present, a projection optical system has accelerated an improvement of its NA; for example, a projection optical system having NA=0.9 has been developed.
It has been reported that influence on imaging performance becomes non-negligible for high NA due to polarization, because the imaging performance becomes different according to polarizations as an incident angle becomes larger. For high NA, the conventional scalar theory cannot predict the imaging performance in a polarization direction of light. Instead, the vector diffraction theory, which precisely regards light as electromagnetic waves, may describe influence on imaging performance due to polarization.
The imaging performance for two-beam interference is more affected by polarization than that for three-beam interference that forms an image when a 0-th order light and ±1st order diffracted beams interfere with each other.
The three-beam interference forms an image through interference among 0th-order light and ±1st order diffracted beams, while an angle is equal to or less than 45° between 0th-order light and one of ±1st order diffracted beams. On the other hand, an angle is close to 90° at most between 0th-order light and one of ±1st order diffracted beams in the two-beam interference.
It is known that mutually orthogonally polarized wave fronts neither interfere with each other nor form an image. Therefore, even when an angle between ±1st order diffracted beams becomes so close to 90° that they hardly interfere with each other, the influence is small because 0-th order light and one of ±1st order diffracted beams interfere with each other. On the contrary, the impact is serious for the two-beam interference when an angle between 0-th order light and one of ±1st order diffracted beams becomes so close to 90° that they hardly interfere with each other.
As discussed, high NA of the projection optical system is necessary for finer patterns, while it is suggested that the imaging performance deteriorates due to polarization of the high NA and the desired pattern cannot be formed. In other words, while high NA of the projection optical system is vital to form fine patterns, an associative phenomenon of deteriorated imaging performance needs to be solved. Nevertheless, few reports have discussed polarization-caused influence on the imaging performance in detail, and an exposure apparatus that may control polarization has not yet been proposed.